The present invention relates to data processing systems. More particularly, the present invention relates to the polishing of substrate materials for magnetic media.
The EDVAC computer system of 1948 is cited by many as the beginning of the computer era. However, well before the introduction of the world""s first stored program computer, the concept of mass storage was well known to early technologists such as Herman Hollerith. In fact, without the advent of mass storage devices there may never have been a computer era. It is no surprise, then, that general advances in computer systems are often closely related to advances in mass storage technology.
Today, one of the most popular forms of mass storage technology is the magnetic disk drive. The disks used in these devices are typically constructed by coating a non magnetic material, such as aluminum, with a magnetic material, such as one of the cobalt alloys. The surface of the magnetic disk is then divided into tiny cells which are magnetically encoded to represent one of the two states of a binary digit (i.e., 1 or 0). The magnetic cells are encoded such that they collectively represent information that can be used by a computer system.
Many of today""s magnetic disk devices further include at least one magneto resonance head which has a write element and a read element. The write element is used to magnetize the cells (i.e., encode the information), while the read element is used to retrieve the information from the magnetic disk. The head(s) is attached to an armature in much the same way as the needle of a record player is attached to a tone arm. Unlike the needle of a record player, however, the head of a conventional magnetic disk device is designed to be aerodynamic. This allows the head to literally xe2x80x9cflyxe2x80x9d across the surface of the disk on a cushion of air without actually contacting the disk surface itself. The altitude at which the head flies (i.e., the distance between the head and the disk) is called the xe2x80x9cfly-height.xe2x80x9d
For cost, space, and access speed reasons, the makers of magnetic disk devices have always been looking for ways to place more and more magnetic cells (i.e., information) on smaller and smaller disks. However, this increase in density hinders the ability of the head to discern one cell from another. This problem was initially resolved by reducing the head fly-height to a point where the head could once again distinguish between individual magnetic cells. Now, however, this constant effort to increase cell density is being hindered by the actual surface roughness of the disk itself. Achievable cell density relates to surface roughness in two key ways. The first and most straightforward relationship between achievable cell density and surface roughness is the aforementioned need to reduce head fly-height. It is easy to envision that a head can pass closer to a smooth disk than it can to a rough disk. The second relationship between achievable cell density and surface roughness has to do with how the magnetic cells are placed on the disk. Magnetic cells encoded on a rough surface require more space than magnetic cells which are encoded on a smooth surface. Therefore, the smoother the surface of the disk, the greater the achievable cell density.
This need to reduce surface roughness has caused increased focus on disk finishing and polishing processes. When a disk substrate is initially created, its surface is very rough. The disk substrate is then polished to reduce this surface roughness. Much like abrasive bathroom cleaners, conventional polishing processes commonly use abrasive particles to polish the disk substrate surface until a certain smoothness is achieved. However, the problem with these conventional processes is that the particles used are too large, too hard, and too sharp. It is easy to envision that at some point in the polishing process, particles of this type are causing surface roughness instead of reducing surface roughness. When this point is reached, the polishing process is merely removing additional material without actually reducing surface roughness.
An intuitive solution to this problem is the use of free abrasive particle mixtures (called slurries) that utilize smaller abrasive particles. There are, however, two prohibitive problems associated with the use of very small particles. First, particles that are very small (i.e., less that one micron in size) tend to clump together (i.e., congeal, coagulate, flocculate, and/or agglomerate) such that the benefit of using small particles is lost. Second, small, soft particles lack abrasiveness to the extent that they are practically useless in conventional polishing processes.
At present, the magnetic disk industry is desperately searching for a finishing process that can further reduce the surface roughness of magnetic disk surfaces. Without such a process, increased cell densities are not achievable.
It is, therefore, a principal object of this invention to provide an enhanced finishing process for disk surfaces.
It is another object of this invention to provide an enhanced Chemical-Mechanical, disk substrate polishing process which uses a slurry that includes soft colloidal particles.
It is still another object of this invention to provide an enhanced slurry which can be used to polish the disk substrate surface by both chemical and mechanical means.
It is yet another object of this invention to provide a hard disk device which has at least one disk substrate with a surface of near atomic smoothness.
It is yet another object of this invention to provide a disk substrate which has a surface of near atomic smoothness.
It is yet another object of this invention to provide a disk substrate which has an enhanced recording surface of near atomic smoothness.
These and other objects of the present invention are accomplished by the substrate independent polishing process and slurry disclosed herein.
The present invention utilizes a combination of chemical and mechanical finishing processes to polish a disk substrate surface to near atomic smoothness. Broadly speaking, the surface of a disk substrate that has been machined (i.e., rough ground) to a predetermined surface roughness is subjected to attack by a chemical formulation (called an attacking agent). The chemical formulation is used to soften the substrate material. Then, the softened material is xe2x80x9cwiped awayxe2x80x9d via mechanical action. As discussed in the background section, mechanical polishing of disk substrates is well known in the magnetic disk art. Damage free chemical polishing of disk substrates, however, is yet to be accomplished because the process parameters have, heretofore, been too difficult to control. Leaving an attacking agent on the substrate for too long causes unacceptable damage to the substrate surface, while leaving an attacking agent on the substrate for too short a period of time defeats the purpose (i.e., material removal).
To overcome this problem, and the inherent problems associated with the use of hard abrasive particles in mechanical polishing, the finishing process of the present invention involves tight control over process parameters and a slurry that comprises soft colloidal particles. When selected appropriately, the relative softness of the colloidal particles combined with their spherical shape and small size make them ideal for damage free removal of the chemically softened material.
In addition to the soft colloidal particles, the slurry used in the present invention is comprised of a polymeric matrix which includes: an attacking agent to soften and loosen the disk substrate material, organic and inorganic stabilizing agents for particle dispersion, and an aqueous carrier that is pH adjusted to be synergistic with the attacking agent.